Municipal solid waste has the following potential negative effects

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Municipal solid waste has the following potential negative effects:

1. Promotion of microorganisms that cause diseases.

2. Attraction and support of disease transmitting vectors like flies and rodents.

3. Generation of obnoxious odors.

4. Degradation of the aesthetic quality of the environment.

5. Occupation of space that could be used for other purposes.

6. Pollution of the environment.

Estimation of quantity

The quantity and general composition of the waste material that is generated is of critical importance in the design and operation of solid waste management system. The following two methods are recommended to estimate the quantity and composition of solid waste.

1. Load-count Analysis

In this method, the volume and general composition of each load of waste delivered to a landfill or transfer station are noted during a specified period of time. The total mass and mass distribution by composition is determined using average density data for each waste category

2. Mass-volume Analysis

This method of analysis is similar to the above method with the added feature that the mass of each load is also recorded. Unless the density of each waste category is determined separately, the mass distribution by composition must be derived using average density values.

Solid waste characterization

The general purpose of solid waste characterization is to promote sound management of solid waste. Specifically, characterization can determine the following:

1. The size, capacity and design facilities to manage solid waste.

2. The potential for composting of biodegradable fraction of the waste stream.

3. The effectiveness of waste reduction programme.

4. Potential sources of environmental pollution in the waste.

Physical Characteristics

Moisture content

The moisture content is expressed on wet or dry basis. The wet percentage moisture (P^) of solid waste is equal to the mass of moisture divided by the total wet mass of the solid. The dry percentage moisture (Pd) of solid waste is equal to the mass of moisture divided by the dry mass of the solid.

Density

The density of mixed solid waste is influenced by the degree of compaction, moisture content and component composition. As individual components of municipal solid waste have different bulk densities. The most important use of the knowledge of the density of solid waste is for the determination of its compacted volume. Densities of solid waste may be expressed on as compacted or as discarded basis. The ratio of the as compacted density pc to the as discarded density is called the compaction ratio r.

Compacting machines are used to reduce the volume of the solid waste before final disposal. Compaction ratios can vary from 2 to 4. Generally, 475 to 594 kg/m3 can be achieved in landfills with a moderate compaction effect. A poorly compacted landfill can achieve only about 297 kg/m3 of compacted density. The as discarded densities of municipal solid waste may vary from 90 to 180 kg/m3, with a typical value of 130 kg/m3.

Combustion Characteristics

Most laboratory analyses are performed on samples of solid waste related to combustion and combustion products. The standard laboratory tests in this category are proximate analysis and heating value.

Proximate analysis

Proximate analysis is a chemical characterization that determines the amount of some surrogate parameters in place of the true chemical content. Surrogate parameters normally determined in proximate analysis are given below:

1. Moisture content'

2. Volatile matter

3. Fixed carbon

4. Ash

The moisture content of solid waste is defined as the material lost during 1 hour at 105°C. Volatile matter is the material driven off as vapour when waste is subjected to a temperature of 950°C for 7 minutes but is prevented from burning. Fixed carbon is the combustible material remaining after the volatile matter is driven off. Ash is the residue remaining after combustion.

Heating value

The heating value of solid waste is measured in kilojoules per kilogram (kJ/kg) and is determined experimentally using a bomb calorimeter. A dry sample is placed in a chamber and burned. The heat released at a constant temperature of 25°C is calculated from heat balance. Because the combustion chamber is maintained at 25°C, water produced in the oxidation reaction remains in the liquid state. This condition enables the maximum heat release and is defined as the higher heating value (HHV).

In actual combustion process, the temperature of the combustion gas remains above 100°C until the gas is discharged into the atmosphere. Consequently the water from actual combustion process is always in the vapour state. The heating value for actual combustion is termed the lower heating value (LHV). The following equation gives the relationship between HHV and LHV.

Solid waste collection

Collection is the first fundamental function of solid waste management. Solid waste collection refers to the gathering of solid waste from places such as residential, commercial, institutional, and industrial areas, as well as public parks. The three basic methods of collection are:

1. Curbside collection

2. Set-out and set-back collection and

3. Backyard collection.

The quickest and most economical point of collection is from curbs using standard containers. In curbside collection, dwellers should keep the container on curbside at the time of collection. The crew simply empties the containers into the collection vehicles and redeposits the container in the original location. Whenever possible, the crew collects from both sides of the street at the same time. Municipal administrative regulations usually specify when the containers must be placed at the curb and also how long they remain after pickup.

The set out and set back collection method consists of the following operations:

1. The set out crew carries the full containers from the residential storage location to the curb before the collection vehicle arrives.

2. The collector crew loads the refuse.

3. The set back crew returns the empty cans.

Backyard collection is usually accomplished by the use of tote barrels. In this method, the collector enters the resident's property, dumps the container into a tote barrel, carries it to the truck and dumps it. The collector may collect refuse from more than one house before returning to the truck to dump. The primary advantage of this method is the convenience to the homeowner. The major disadvantage is the higher cost.


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